Purified enterotoxin (protein, MW equals 35,000) from the bacterium Clostridium perfringens possesses no detectable protease or phospholipase activity yet rapidly depresses (but does not abolish) a hormonally induced level of amino acid transport in primary cultures of adult rat liver parenchymal cells (hepatocytes). Five rat hepatoma cell lines have also been tested: one is sensitive (McA-RH 7777), one has reduced sensitivity (MH1C1), and three appear resistant (H-35, HTC and McA-RH 8994) to the toxin. These findings indicate that cultured normal and malignant liver cells represent useful models for studying the mechanism of action of C. perfringens enterotoxin. Using sensitive cells we will investigate the effect of this toxin on membrane integrity, intracellular sodium ion levels, and sodium-independent transport systems. We will also investigate toxin binding to sensitive and resistant cells to determine whether there exist specific binding sites for toxin on the cell membrane and, if so, whether specific binding correlates with biological activity.